1. Field of the Invention
This invention relates to a process for producing ferromagnetic iron powder which has utility for high-density magnetic recording media such as magnetic tapes, magnetic drums and magnetic discs.
2. Description of the Prior Art
It has heretofore been known that ferromagnetic iron powder can be produced by heating and reducing an oxyhydroxide or oxide such as acicular .alpha.-FeOOH or .alpha.-Fe.sub.2 O.sub.3 (hereinafter abbreviated as "acicular goethite or the like"). However, such a conventional process is inevitably accompanied by breakup, fragmentation and sintering of the raw material, goethite or the like upon heating and reducing same. As an improvement to the above-described prior art process, it has been proposed in Japanese patent publication No. 42832/1979 to obtain ferromagnetic metal powder by dipping its corresponding oxide or oxyhydroxide in an aqueous solution containing boron oxide or a borate dissolved therein and then heating and reducing the thus-dipped oxide or oxyhydroxide.
When the latter process is followed, a great deal of energy is required because a large volume of water, which contains boron oxide or borate, has to be removed by evaporation. Furthermore, the boron oxide or borate dissolved in water tends to move together with water toward the evaporation interface upon driving off the solvent and the concentration distribution of boron oxide or the borate within the resulting powder becomes unavoidably uneven, leading to another disadvantage that the powder has to be heated and reduced while being of uneven concentration distribution. As a result, the magnetic properties of the thus-obtained powder deteriorate.
The above-described phenomenon that boron oxide or the borate moves to the evaporation interface upon the removal of the solvent by evaporation will be described in further detail. The concentration of boron oxide or the borate in a surface layer increases due to, for example, the movement of a filter layer to the surface phase, the movement of filter cake onto the surfaces of lumps or the like, thereby deleteriously affecting the uniformity of the ferromagnetic iron powder to be obtained.